Don battling dry air and wind shear

Tropical Storm Don continues to be an unimpressive low-end tropical storm as it continues northwest towards the Texas coast. Don formed yesterday afternoon from an African tropical wave that moved into the Gulf of Mexico under a region of low wind shear. Don's formation date of July 27 is nearly a month ahead of the usual August 23 date for the arrival of the season's fourth named storm of the year. There is currently no hurricane hunter airplane in Don, and a new airplane is not due in the storm until tonight. The last center fix at 1pm EDT found surface winds of 45 mph and a central pressure of 1005 mb, a 4 mb rise from earlier this morning. Water vapor satellite images show a region of dry air to the northwest of Don, over the western Gulf of Mexico. Wind shear as diagnosed by the University of Wisconsin CIMSS group and the SHIPS model show a moderate 5 - 15 knots of shear from strong upper level winds out of the north. This shear is creating problems for Don by injecting dry air into the system. Visible satellite imagery from early this afternoon showed the presence of surface arc-shaped clouds expanding outwards to the north from the center of Don. These type of clouds are a sign that the storm is struggling with dry air. When dry air at middle levels of the atmosphere gets injected into thunderstorms due to wind shear, the dry air tends to create strong downdrafts that rob the storm of moisture. These downdrafts spread out at the ocean surface and create arc-shaped surface cumulus clouds.

Figure 2. The latest drought map for Texas shows that over 75% of the state is in exceptional drought--the highest category of drought. Image credit: U.S. Drought Monitor.

Forecast for DonThe big question for Don is, will it bring significant rains to Texas? According to the National Climatic Data Center, the six-month period ending in June 2011 was the driest on record. Average rain between January and June was more than eight inches (203 millimeters) below average in Texas, and the state experienced record heat between April and June. The heat and lack of rain have brought exceptional drought--the highest category of drought--to over 75% of the state. Don has the potential to bring some decent drought-busting rains to the state. If Don can expand in size and intensify to a 50 - 55 mph tropical storm, it has the capability to bring hundreds of millions of dollars worth of beneficial rains to the state. We don't want Don to stay in its current state, which is too small and weak to bring significant rains to Texas. If Don follows the current NHC forecast, which brings the storm up to a moderate-strength tropical storm, that would be just right. Don's small size makes it prone to dry air and wind shear, though, and it is uncertain whether the storm can overcome these problems enough to become a significant rain maker. NHC gave Don a 12% chance of intensifying into a hurricane in the 11am advisory, which is a reasonable forecast, since Don is running out of time to get its act together in time to become a hurricane. None of the computer models is predicting Don will become a hurricane.

For those of you wondering about your odds of experiencing tropical storm force winds, I recommend NHC's wind probability forecast. The 11 am version of this forecast shows that Port O'Connor, Texas has the highest chance of tropical storm-force winds (39+ mph): 45%.

New hurricane archive search featureThe autocomplete entities in the wunderground search box has been extended to include hurricanes, so you can now search for a storm by name, year, or basin. Here are some examples in case you feel like exploring your new options:

Quoting TampaSpin:The Timing of this is SCARY as heck.......this reminds me of the "A" storm with timing........the high breaks down just to pull it North then the HIgh builds back in.........I DON"T LIKE THE LOOKS OF THIS AT ALL!!!!!

Quoting Levi32:Lowest surface pressure was not found at the center of flight-level winds, but rather north of it. This means that Don's circulation is still vertically tilted southward.

So Levi, the EURO has been right?From the NHC-Discussion:HIGH PRESSURE RIDGING OVER THE SOUTHERN UNITED STATES WILL PROBABLY STEER THECYCLONE ALONG THAT GENERAL COURSE UNTIL LANDFALL...PERHAPS EVEN ALITTLE MORE TO THE LEFT AS THE RIDGE BUILDS. THE GUIDANCE IS INGOOD AGREEMENT...ALTHOUGH THE USUALLY RELIABLE ECMWF MODEL ISFARTHER TO THE NORTHEAST. AN EXAMINATION OF THE ECMWF FIELDSINDICATES THAT THE MODEL DECOUPLES THE LOW- AND MID-LEVELCENTERS...ALLOWING DON TO MOVE MORE TO THE NORTHWEST. THIS DOESNOT SEEM LIKELY AS ENVIRONMENTAL CONDITIONS ARE NOT EXPECTED TOCHANGE SIGNIFICANTLY FROM NOW UNTIL LANDFALL. THUS THE NHCFORECAST WILL STAY CLOSE TO THE DYNAMICAL MODEL CONSENSUS AND THEPREVIOUS FORECAST.

Movement will continue to jump around before landfall, as it has all night. The reason for this is that whenever a convective burst fires near the center, it is pulled nearly underneath and turns more westward. Once each burst weakens, the surface center is more susceptible to being pulled out to the NW by the low-level flow. These stepped increments of movement have been evident for the last 12 hours or so.

You don't happen to remember the street that was on do you? I was one of the idiots that didn't evac.

just ck'd internet...surge for hugo averaged 20 ft (there is a pole with a marker in awendaw with a line where the surge came to)...that would put it at the 3rd story of a home as well... and the school that was an evac shelter was 10 ft above sea level, the error was in a transcript that was sent out saying it had a 20ft elevation...

I did home inspection in the Bay St Louis, MS area after Katrina. I saw a 30+ foot sailboat on top of a 2 story home and the high water mark in a home I inspected was in the 3rd floor of the 3 story residence.

You don't happen to remember the street that was on do you? I was one of the idiots that didn't evac.

Quoting HimacaneBrees:From Levi's tidbit"The wave has a closed low, though broad, and in proximity to dry air to the north and west. The wave looks a lot better-defined than pre-Don did over in that area, and this system may have even better conditions ahead of it west of 55W than Don had."

If it has a closed low then should it be called a depression? Or is there more that needs to happen for it to be classed as a depression? Wind speed, pressure, etc.?

It needs to show clear separation from the ITCZ and become more well-defined. The NHC doesn't like classifying broad circulations, closed or not.

Quoting HimacaneBrees:From Levi's tidbit"The wave has a closed low, though broad, and in proximity to dry air to the north and west. The wave looks a lot better-defined than pre-Don did over in that area, and this system may have even better conditions ahead of it west of 55W than Don had."

If it has a closed low then should it be called a depression? Or is there more that needs to happen for it to be classed as a depression? Wind speed, pressure, etc.?

Needs to look better i think, and Pre Don had alot of shear close to the islands.

From Levi's tidbit"The wave has a closed low, though broad, and in proximity to dry air to the north and west. The wave looks a lot better-defined than pre-Don did over in that area, and this system may have even better conditions ahead of it west of 55W than Don had."

If it has a closed low then should it be called a depression? Or is there more that needs to happen for it to be classed as a depression? Wind speed, pressure, etc.?

Quoting FLWeatherFreak91:Keep one thing in mind with systems forecasted to take a track over the antilles then get lifted north by a trof: If the system experiences significant weakening over the islands, many times the system will become weak enough to slip under the trof against model consensus. We saw this with a couple of storms in 2004.

Yes, they have the tendency to regenerated after the islands. Good point.

my 3rd cup was b4 7am est lol...work 6am-6pm today...yay me but agree...set up NOT GOOD! They usually don't fly into storms that far away...wonder if it will spin up enuf visually for them to classify it b4 Monday...5 named storms by end of July would be absolutely CRAZY

Yea I work from 12pm to 8:30pm so I get to see whats happening.... Although I hate being on call because I would be in TX right now waiting on a Storm :o)

I did home inspection in the Bay St Louis, MS area after Katrina. I saw a 30+ foot sailboat on top of a 2 story home and the high water mark in a home I inspected was in the 3rd floor of the 3 story residence.

the people that went to a shelter at a school...they were in the gymnasium for hugo...water came in...they had to crawl up into the ceiling of the gymnasium and this was an evac shelter...i saw things during/after Hugo i hope i NEVER see again...not home to get my book out with all the data from Hugo...will see if i can get it later

The Timing of this is SCARY as heck.......this reminds me of the "A" storm with timing........the high breaks down just to pull it North then the HIgh builds back in.........I DON"T LIKE THE LOOKS OF THIS AT ALL!!!!!